The present invention generally relates to electric machine drive systems and deals more particularly with the estimation of bus voltage for use in both fault protection and machine control.
In connection with modern electric machine drives, knowledge of the DC bus voltage used to drive a machine is employed for both fault protection and control. In electric and hybrid electric vehicle applications, for example, this knowledge is also crucial in determining the state of charge of a battery used to power the vehicle. In order to measure the bus voltage for the vehicle drive system, analog circuits are typically employed, however this method of bus voltage estimation requires unnecessary hardware and suffers from other shortcomings.
Within the last decade, significant advancements have been made in the area of sensorless control of electric machines. In order to avoid the use of dedicated sensors for determining rotor speed and position, methods have been developed for estimating the rotor or flux position by injecting a high frequency voltage or current signal into the machine, referred to as a carrier signal. These methods yield information used for regulating torque and speed without the need for a position or flux sensor. The carrier signal injection can be achieved by injecting either current or voltage, and in the description that follows, only voltage injection will be discussed, although the same estimation routines can be applied to current injection.
The elimination of the position or flux sensor using sensorless control results in lower costs, increased reliability, and potentially increasing the performance of the system. Cost reduction is achieved as a result of the elimination of the physical sensor and its associated cabling and connectors. By decreasing the amount of connections and hardware complexity, the reliability of the motor and its regulation system is increased. Because low resolution sensors are normally used in automotive vehicle applications, the use of sensorless control is capable of substantially increasing system performance.
The injection of a positive rotating carrier voltage signal into a motor, in addition to the fundamental excitation used to drive the motor, induces a current in the motor that is comprised of two components, a positive rotating sequence and a negative rotating sequence. The negative rotating sequence may contain spatial information concerning the position of the rotor or the flux in the motor if the machine is salient, whereas the positive sequence contains information relating to the bulk properties of the motor and drive system.
Although a number of techniques have been devised for sensorless control of electric machines, none have also been used for estimating the bus voltage.
It is an object of the present invention to provide a method of estimating the bus voltage of an electric machine drive system by injecting a carrier signal into the system and determining the magnitude of the positive sequence current component induced in the system by the carrier signal.
In accordance with the primary object of the invention, in a preferred embodiment, the method comprises injecting a carrier signal into the machine; determining the magnitude of the positive sequence current component induced in the machine by the carrier signal; estimating the bus voltage of the system using the determined magnitude of the current component, and altering the commanded voltage used to drive the machine based on the estimated bus voltage.
An advantage of the present method is that the bus voltage may be estimated without the need for physical voltage sensors by interpreting the positive sequence component of current induced in the machine by the carrier signal. An added advantage of the invention is that the method may be employed with existing sensorless control systems for electric machine drives.
These and other features and advantages of the present invention may be better understood by considering the following details of a description of a preferred embodiment of the invention. In the course of this description, reference will frequently be made to the attached drawings.